Ben Eater 6502 Kit — Day 15

in Project HOPE2 months ago

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The first version of the Hello World program in bare metal 65C02 assembler was 342 bytes long. That's quite a lot for such a simple assembler program. We can certainly do better. Since the RAM is not yet fitted subroutines are not yet an option. But we can do loops:

.pc02
.listbytes  16
.pagelength 66
.case     -

.macpack    generic
.include    "VIA.inc"
.include    "LCD.inc"

.segment    "RODATA"

Message:    .byte "Hello World!"
Message_Len =  * - Message

.segment    "CODE"

Do_RES:     LDX  #$FF
    TXS

    Set_A #%11100000  ; Set top 3 pin as output
    Set_B #%11111111  ; Set all pins as output

    Control_LCD #%00111000  ; Set 8-bit mode; 2 line display; 5×8 font
    Control_LCD #%00001110  ; Display in; cursor on; blink off
    Control_LCD #%00000110  ; Increment and shift cursor; don't shift display
    Control_LCD #%00000001  ; Clear Display

    LDX  #$00
Loop:     Data_LCD {Message,X}  ; Write next character to Display
    INX
    CPX  #(Message_Len)  ; Repeat loop until X ≥ message lenght
    BLT  Loop

    STP     ; Stop Processor

Do_NMI:     RTI

Do_IRQ:     RTI

.segment    "HEADER"
.word     Do_NMI
.word     Do_RES
.word     Do_IRQ

A lot shorter. So what did I do?

  1. Moved the macros for the VIA and LCD out into include files. That way I don't need to repeat them in every new iteration of the program.
  2. Cleared the display. Without the text will not appear right at the beginning of the display. Do compare with the previous blog and see the difference.
  3. Stored the text as string in the RODATA (read only data) segment. This segment is written to the ROM after the CODE segment but before the HEADER segment.
  4. Replaced the repeated Data_LCD calls with a loop using the index register X as index to the current character. I used a simple count up loop as it's not that difficult to have the assembler calculate the length of the string instead of using a terminator character.

The result: The program is down to 135 bytes.

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You find the source code for the program with makefile, linker configuration file, include files and assembler source code on GitLab: 6502Tutorial — Tools/Create_LED